1. Field of the Invention
The present invention relates to an optical deflection device.
2. Description of the Related Art
Laser scanning type projection image display devices which display a two-dimensional projection image by scanning using a laser beam light source are more expensive than display devices using a two-dimensional display element such as a liquid crystal display or the like, but provide various advantages in that they have excellent color reproducibility, low power consumption and the like.
The head-mounted display (HMD) shown in FIG. 23 is an example of the conventional art of a scanning type projection image display device. FIG. 23 briefly shows an HMD optical system viewed from above. Reference number 101 refers to a main scanning mirror for scanning light in a horizontal direction. As can be understood from the structure viewed from the horizontal shown within the circle, a laser beam emitted from a laser light source 102 is modulated by the projection image signals, and condensed by a condenser lens 103, then impinges the main scanning mirror 101.
Reference number 104 refers to a sub scan mirror for scanning the light from the main mirror 101 in a perpendicular direction. Reference number 106 refers to a concave surface mirror for projecting a virtual image to the pupil of an observer. Reference number 105 refers to a half-mirror which participates with the concave surface mirror 106 to reflect the light emitted from the subscan mirror 104, and participates in transmitting the light from the concave surface mirror 106 to the pupil of the observer. The main scanning means 101 is formed at a position approximately conjugate to the pupil position of the observer through the concave surface mirror 106.
In this scanning type projection image display device, the number of scan lines per screen must be increased to provide a high resolution image. For this reason the scanning frequency of the scanning means such as a scanning mirror must be very high. For example, in order to display a projection image of 1,000 horizontal scan lines, a scanning frequency in the horizontal direction of 60 kHz is required.
In an optical deflection device using a galvano mirror and a rotating polygonal mirror using an electromagnetic motor, it is difficult to obtain the aforesaid high frequency. However, an optical deflection device having a resonance mirror using spring resonance is known to be effective in obtaining a high frequency.
In an optical deflection device having a resonance mirror, the light must scan at a large amplitude to display a projection image at a wide field angle. Amplitude relative to energy is determined by the amplitude magnification and natural oscillation frequency determined by the spring constant and moment of inertia of the oscillation system. The maximum oscillation magnification is obtained when the natural oscillation frequency of the oscillation system matches the drive frequency of the drive system. That is, a large oscillation can be obtained using little energy by setting the natural oscillation frequency near the drive frequency to increase the oscillation magnification. Accordingly, in a typical optical deflection device, the natural oscillation frequency of the oscillation system approximately matches the drive frequency of the drive system.
The natural oscillation frequency of the oscillation system fluctuates depending on temperature. This change occurs because the spring constant changes if the flexibility of a spring is changed by temperature. In optical deflection devices, for example, when a specific frequency projection image is presented, the ambient temperature changes and the natural oscillation frequency changes so as to disadvantageously reduce the amplitude so that a maximum amplitude magnification cannot be obtained. U.S. Pat. No. 5,557,444 and Japanese Laid-Open Patent No. HEI 7-49462 disclose art to eliminate this disadvantage.
The optical deflection device disclosed in U.S. Pat. No. 5,557,444 arranges a mirror in the center of a torsion spring, to adjust the spring tension by combining materials having different coefficients of expansion, so as to make the natural oscillation frequency adjustable.
The optical deflection device disclosed in Japanese Laid-Open Patent No. HEI 7-49462 detects the temperature in the vicinity of the oscillation system, and controls the drive frequency based on the detection result. In this instance, even if the natural oscillation frequency of the oscillation system changes due to temperature fluctuation, the drive system can be driven at a drive frequency in accordance with the change. In this way the oscillation system is controlled so as to resonate.
In a high-speed resonance state, the adjustment range becomes extremely narrow, and fine adjustment is difficult. In the adjustment method disclosed in U.S. Pat. No. 5,557,444, fine adjustment is difficult and a desired natural oscillation frequency cannot be obtained.
When the drive frequency is changed as in the optical deflection device of Japanese Laid-Open Patent No. HEI 7-49462, it becomes necessary to change the frequency of the projection image signals, thereby disadvantageously increasing the cost. In conventional optical deflection devices, only one natural a oscillation frequency typically can be obtained, which is disadvantageous from the perspective of general purpose utility.